Not applicable.
1. Field of the Invention
This invention relates, generally, to the manufacturing of polymer impregnated specialty papers, and particularly to a method and device for impregnating a cellulose web with a molten polymer.
2. Description of Related Art
The growth of the fast food industry over the past several decades has been matched by an increase in the demand for paper products that are associated with fast food items, such as beverage cups, coffee filters, paper wrappers, and paper food containers. These paper products are fashioned from specially engineered papers (xe2x80x9cspecialty papersxe2x80x9d), which are produced to suit both the commercial needs of fast food vendors as well as the domestic needs of families in their homes. Among these specialty papers, food wrappers comprise a generous portion of the food paper market.
Food wrappers are traditionally made from waxed paper, which may be imprinted with the name or logo of the vendor whose product is being sold. Waxed paper is commonly used to wrap various food items, including, among others, sandwiches, tacos, gyros, french fries, biscuits, onion rings, popcorn, and chicken tenders. Unfortunately, the ability of most waxed paper food wrappers to resist grease or oil is marginal at best, especially when the grease or oil is hot. In fact, if grease or oil, whether hot or cold, is allowed to remain in contact with food grade waxed paper for any substantial length of time, the grease or oil will penetrate the paper and come into contact with the hands, clothing, or surroundings of the user.
Therefore, there exists a need for an improved paper food wrapper that has improved resistance to oil and grease emanating from the food that is contained in the wrapper, while at the same time retaining flexibility and the ability to receive printed material on its surface. More importantly, the improved paper food wrapper needs to be inexpensive and easy to manufacture. In order to keep production costs at a minimum, the paper from which the improved food wrappers are generated should be manufactured as part of a continuous on-machine process, rather than a batch process, which consumes more time and resources.
Preferably, the device and process for making an improved food wrapper should be located at the end of a traditional paper manufacturing process as an additional and optional step in the specialty paper production process. The total process should flow as a single manufacturing operation without interruption from the time the paper web is formed from pulp fibers to the time the paper web encounters the finishing process that imparts superior oil and grease resistant qualities to the web. The process should not require the manufacturer to stop the machine before the paper web completes the finishing process. Moreover, the device should be quick and simple to operate, should eliminate unnecessary steps, and should not require the manufacturer to dedicate substantial time or resources to the production effort.
In the art of manufacturing specialty papers, a cellulose or synthetic paper web may be treated with various compounds to improve certain characteristics of the paper that is treated, including strength, durability, printability, and resistance to water, oil, grease, and ultraviolet radiation. One such compound that can be applied to paper is paraffin wax.
Waxed paper is produced in a number of ways, including the application of a paraffin film to a paper web using a mechanical roller. The paper produced by this particular method is a wax xe2x80x9ccoatedxe2x80x9d paper. The term xe2x80x9ccoatedxe2x80x9d is used to describe this process because the paraffin film is in contact with only one side of the paper web and the depth of penetration of the wax into the paper fibers is only superficial.
Polymers that are capable of withstanding higher temperatures than paraffin have also been applied to a cellulose paper web using a method known as extrusion. In the extrusion method, a polymer is heated to a semi-solid state and then pushed through a die to form a semi-solid film, which is then fed into a nip where the film is pressed against a paper web. The features of the coated paper generated using this method are similar to those of wax coated paper, although the polymer coated paper may have better resistance to grease penetration.
While papers treated using the extrusion method may have certain advantages over wax coated papers, they still have inferior resistance to oil and grease due to the fact that the extruded material does not permeate throughout the paper, but adheres to only one side. Therefore, if the untreated side is exposed to grease or other liquid, the cellulose fibers will be weakened and the structural integrity of the paper product will be compromised. While extruded layers could be applied to both sides of a paper web, the process would be expensive and difficult to achieve at a commercial level. Moreover, introducing an additional layer would add unwanted weight, rigidity, and bulk to a paper product that is intended to be light, flexible and thin in order to conform to the shape of the food around which it is wrapped. Finally, even if the grease, oil, or other liquid contact the side of the paper containing the extruded material, the integrity of the extruded barrier may be compromised by the breaking of the film that commonly occurs when such paper is creased or folded.
The extrusion method is also inferior because it requires the use of sophisticated precision equipment. The material that is to be extruded must be heated to a temperature that is high enough to transform the material from a solid state to a pliable, semi-solid state. However, the temperature of the extruder should not be so high as to make the material a flowable liquid, which would be incapable of forming the film that is ultimately applied to the paper web. In addition, once the optimum temperature is achieved, the semi-solid material must be pushed through a die at a rate of speed sufficient to match the rate of speed at which the extruded film is pressed against the web. In practice, the extrusion method has proven difficult to implement in an on-machine process, where the web encounters the extruded film at a high rate of speed.
Another method of treating paper in order to improve grease resistance and durability is the impregnation method. In this method, a molten liquid compound is applied to a cellulose or synthetic web. The molten compound permeates the web and adheres to the individual cellulose or synthetic fibers. The paper produced by this method is superior to papers with wax film or extruded layers, which merely bond to the surface of the web and do not coat the fibers beneath the surface of the paper.
The impregnation method has been used exclusively in connection with the application of resinous compounds or aqueous polymeric dispersions (formed when a polymer is mixed with water). The disadvantage of using these resinous or aqueous compounds to impregnate a paper web is that each compound requires a subsequent processing step before the paper product may be stored on a winding reel. Resinous compounds must be allowed to cure over a period of time. The curing process may also involve the application of heat. After an aqueous polymeric dispersion is applied to a paper web, the product must undergo a drying period, in which the water must be evaporated away from the polymeric compound. This evaporation usually occurs in a drying step. Both of these secondary processing steps consume time and resources, and consequently reduce output volume and increase the price of the finished product.
What is needed is a polymer impregnation process and device for carrying out the process that overcome the shortfalls of the processes and devices that are currently known in the art.
It is an object of the present invention to provide a device for impregnating a cellulose paper web with a molten polymer.
Another object of the present invention is to provide a device for impregnating a cellulose paper web with a molten polymer that produces a polymer impregnated paper with superior resistance to grease.
Another object of the present invention is to provide a device for impregnating a cellulose paper web with a molten polymer that can be efficiently utilized as an on-machine process, rather than as a batch process.
Another object of the present invention is to provide a device for impregnating a cellulose paper web with a molten polymer that eliminates the need for a subsequent drying or curing step.
Another object of the present invention is to provide an on-machine method for impregnating a cellulose paper web with a molten polymer.
Another object of the present invention is to provide a polymer impregnated paper obtained by the method described hereinafter.
A device and method for impregnating a paper web with a molten polymer are provided. The device and method utilize a scientific phenomenon known as xe2x80x9cwickingxe2x80x9d to penetrate or xe2x80x9cimpregnatexe2x80x9d a web of cellulose fibers with a polymer that has been heated to a molten state. Wicking, which is the process by which a liquid permeates a fibrous web and adheres to the individual fibers, can vary in degree, depending on the temperature, viscosity, and surface tension of the penetrating liquid. As a general rule, the longer the penetrating liquid remains above its melting point, the greater the degree of wicking that occurs. Once the liquid cools to a temperature below its melting point, the liquid begins to solidify and wicking ceases. Of course, some substances require curing or drying beyond the mere solidification of the substance, but these steps are unnecessary in the present invention.
The device includes, as part of an on-machine process, an ordinary paper web composed of cellulose or synthetic fibers (along with any additives such as clays, dyes, fillers and other substances common in the art), a polymer applicator, a hot reel, and a winding reel. The web, which is created through a series of processes located upstream of the subject invention, first passes over the polymer applicator, where an applicator roller, part of which is immersed in a molten polymer bath, deposits a quantity of molten polymer onto the surface of the web. After the molten polymer has been applied by the polymer applicator, the web passes an optional distributing means, which evenly distributes the molten polymer onto the surface of the web and removes any excess polymer.
The web is next transferred to a hot reel, which is a metal drum heated to a temperature sufficient to maintain the molten polymer above its melting point. Because the polymer is maintained in liquid state while being applied to the web and during the time for which it is in contact with the hot reel, significant wicking of the polymer into the web occurs.
After the web passes the hot reel, it is transferred to a winding reel, where the paper web is wound and stored for further processing or shipment. For a period of time after the web is transferred to the winding reel, the polymer remains in a molten state and will continue to wick throughout the web. Once the polymer cools below its melting point, wicking will cease and the process is complete.
Depending on the temperatures of the various process components, the user can select the degree of wicking preferred. A higher temperature will maintain the temperature of the polymer above its melting point for a longer period of time and thereby produce a paper with fibers that are completely permeated and coated with polymer. A lower temperature will produce a paper with fibers that are minimally coated or superficially coated to a minimal depth.
An alternative embodiment of the invention includes replacing the hot reel with a cold reel or cold rolls to reduce the temperature of the molten polymer after application and to control the degree of wicking.
Another embodiment of the invention includes varying the respective velocities of the applicator roller and the web in order to increase or decrease the amount of polymer that is deposited on the web.
Another embodiment of the invention includes the addition of guide rollers to increase or decrease the amount of surface contact that the web has with the applicator roller and the hot reel.
Another embodiment of the invention includes a polymer applicator with a polymer recirculation means.
Another embodiment of the invention includes a polymer applicator that is capable of simultaneously depositing molten polymer onto both sides of the web.
An advantage of the present invention is that the device and process produce a polymer impregnated paper with superior resistance to oil and grease.
A further advantage of the present invention is that the web may be impregnated as part of an on-machine process, rather than a batch process.
A further advantage is that the invention eliminates the subsequent curing or drying steps required for resinous compounds or aqueous polymeric dispersions.
These and other objects, advantages, and features of this invention will be apparent from the following description.